1. Field of the Invention
The present invention relates to a method and circuit for measuring the Q-factor of an optically amplified system. More particularly, the present invention relates to a circuit for determining the performance characteristics of a digital, optical waveform.
2. Description of the Related Art
In any transmission system, including optically transmitted systems, it is desirable to know the accuracy of the transmitted data at the receiver, i.e., the end of the system. In a digital system, the transmitted signal comprises a plurality of 1""s and 0""s, i.e., a plurality of high and low signals. Thus, errors in transmission occur when these 1""s and 0""s are not properly identified by the receiving circuit.
The digital values xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d each have an ideal voltage associated with it depending upon the parameters of the transmission circuit. Since no system is completely ideal, however, the actual 1""s and 0""s being transmitted will run through a range of voltages around the ideal voltages. For this reason, the ideal voltage can also be referred to as a mean voltage xcexc, since it is the average voltage transmitted for a given digital value. The distribution of these voltages will either be Gaussian or at least a close approximation of Gaussian for the range of interest.
This means that the distribution of voltages transmitted as a particular value, i.e. xe2x80x9c1xe2x80x9d or xe2x80x9c0,xe2x80x9d will fall into at least an approximation of a Bell curve, as shown in FIG. 1. As shown in FIG. 1, for a given digital value, i.e. xe2x80x9c1xe2x80x9d or xe2x80x9c0,xe2x80x9d half of the transmitted voltages will fall above the mean voltage xcexc, half will fall below the mean voltage xcexc, and a majority will fall within three standard deviations 3"sgr" of the mean voltage xcexc.
In a given system, therefore, it is necessary to set some threshold voltages for the 1""s and 0""s, i.e., HIGH and LOW thresholds, to allow a decision circuit to determine what will be identified as received 1""s and 0""s. The closer these thresholds are to the mean voltage xcexc, the greater the error rate will be, and the farther away, the lower the error rate will become.
The accuracy of the transmitted signals can be determined by a statistic called the bit error rate (BER). The BER is the number of errors per bit transmitted, and depends upon the decision threshold. Another indicator of the accuracy of transmission can be given by the transmission""s Q-factor. The Q-factor is an indicator of the signal quality at the decision circuit.
While the BER is easy to understand, the Q-factor is generally considered a more useful indicator of the accuracy of a transmission circuit, because it can be used to characterize the signal quality under conditions in which it is not practical to measure the BER. For this reason, it is preferable to determine a circuit""s Q-factor rather than its BER. The Q-factor is related to the BER at th e optimal threshold setting by the following formula:                     BER        =                                            1              2                        ⁢                          xe2x80x83                        ⁢                          erfc              ⁡                              (                                  Q                                      2                                                  )                                              ≈                                    1              2                        ⁡                          [                                                1                                                            (                                                                        2                          ⁢                          π                                                                    )                                        ⁢                    Q                                                  xc3x97                                  ⅇ                                      -                                          xe2x80x83                                        ⁢                                                                  Q                        2                                            2                                                                                  ]                                                          (        1.        )                                BER        ≈                              1            2                    ⁡                      [                                          1                                                      2                    ⁢                    π                                                              xc3x97                              1                Q                            xc3x97                              ⅇ                                  -                                      xe2x80x83                                    ⁢                                                            Q                      2                                        2                                                                        ]                                              (        2.        )            
As a result, it is possible to determine the Q-factor of a signal by first measuring BER versus threshold for both xe2x80x9c1sxe2x80x9d and xe2x80x9c0sxe2x80x9d, and then fitting the results to extract the Q-factor.
This relationship is helpful, since the BER is more readily measured than the Q-factor. To measure the BER of a signal a measuring circuit need only monitor an incoming circuit for errors and determine how frequent the errors are. The accuracy of an error count is roughly {square root over (N)}, so a rule of thumb is that 10 errors has an uncertainty of 3%.
In conventional optical transmission systems, BERs of 10xe2x88x9215, i.e., one error per 1,000,000,000,000,000 bits transmitted, are typical. These low BERs lead to one significant problem. Given the small number of errors, it is extremely difficult to actually measure the BER of an optical system in an efficient manner. Since an accurate BER measurement requires the measuring circuit to detect ten individual errors it is necessary to run the measuring circuit for a sufficient period of time for ten errors to pass through. This means that with a BER of 10xe2x88x9215, the detection circuit would have to actually detect 1016data bits before it detected the ten errors required for an accurate BER measurement. For an optical system that can transmit 2.488xc3x97109bits per second (i.e., OC48), it would take nearly 4xc3x97106seconds, or 46 days, for ten errors to be detected, and thus for the BER to be accurately determined.
This is too long a time for any effective testing circuit to employ such a method. As a result, it is extremely difficult to measure the true BERs for the threshold voltages used in optical transmission systems, and thus similarly difficult to determine the systems""Q-factors. It is therefore desirable to have a way of easily determining the BER or Q factor without having to wait over a month for each test sample.
One possible method of estimating BER was suggested in detail in xe2x80x9cMargin Measurements in Optical Amplifier Systems,xe2x80x9d by Neal S. Bergano, et al., IEEE Photonics Technology Letters, Vol. 5, No. 3, Mar., 1993, (xe2x80x9cBergano et al.xe2x80x9d) the contents of which are herein incorporated by reference. Bergano et al. observes that high values of BER can be easily measured and plotted against their respective threshold voltages. If several measurements are taken of high BER values and plotted against the median voltage on a logarithmic scale, the resulting curve is a close approximation of a straight line. Bergano et al. then suggests plotting one line for the threshold for 1""s transmitted through the optical system and another line for 0""s transmitted through the optical system.
The point at which these two lines intersect will be the point where the optimal threshold voltage is for marking the difference between 1""s and 0""s, and will show the BER for that threshold voltage. In this way, the BER can be quickly determined for an ideal threshold voltage, even if the time required to actually confirm that BER would be great. Using Bergano et al.""s method, measurements need only be taken for several larger BER""s, which will take a dramatically shorter amount of time.
However, Bergano et al. does not suggest any circuitry for implementing this method, nor does it address the problems inherent in implementing the method into a physical circuit. It is therefore desirable to provide a functional circuit that can provide an accurate reading of the actual BER for both incoming 1""s and 0""s and accurately extrapolate an optimal threshold voltage and associated BER value for that threshold. It is also desirable to provide a functional circuit that can determine the associated Q-factor for the BER associated with the optimal threshold voltage.
It is thus an object of the present invention to provide a circuit for determining the Q-factor an optical communication system in an accurate fashion.
In particular, it is an object of this invention to provide a test instrument that will measure the Q-factor of an optical communication system by first determining the optimal BER and threshold voltage of the system, and then using the optimal BER to calculate the optimal Q-factor.
In accordance with these objects, a Q detection circuit is provided, comprising: a first variable attenuator for attenuating a received optical signal in response to a first attenuator control signal, a first optical-to-electrical converter for converting a first portion of the attenuated optical signal into an electrical data signal, a second optical-to-electrical converter for converting a second portion of the attenuated optical signal into a first power indication signal, a decision circuit for detecting high and low data bits in the electrical data signal based on a plurality of threshold voltage signals, and for providing decision signals indicative of the results of these determinations, an error monitoring circuit for receiving the decision signals, determining the bit error rate of the incoming optical signal for the plurality of threshold voltages, and providing bit error rate signals, and a microprocessor for receiving the power regulation signal and the bit error rate signals, and for generating a first attenuator control signal and a plurality of threshold a voltage signals. In this Q-detection circuit, the variable attenuator operates to attenuate the received optical signal such that it is at an optimal input level for the operation of the first optical-to electrical converter. Also, the microprocessor determines an optimal bit error rate and an optimal Q-factor for the incoming signal based on the bit error rates of the incoming optical signal for the plurality of threshold voltages.
A method is also provided for determining the optimal Q-factor of an optical signal containing a plurality of data bits, each of the plurality of data bits having a value of xe2x80x981xe2x80x99 or xe2x80x980,xe2x80x99 the method comprising the steps of: receiving the optical signal, attenuating the optical signal to a desired intensity, converting the attenuated optical signal to an electrical signal, determining the value of each of the data bits for each of a plurality of threshold voltages, determining a 1-bit error rate for identifying bits having a value of xe2x80x981xe2x80x99 in the step of determining the value of each of the data bits, determining a 0-bit error rate for identifying bits having a value of xe2x80x980xe2x80x99 in the step of determining the value of each of the data bits, approximating a 1-bit error line of the logarithm of the 1-bit error rate versus the threshold voltage, approximating a 0-bit error line of the logarithm of the 0-bit error rate versus the threshold voltage, determining an intersection point at which the 1-bit error line and the 0-bit error line cross, determining the ideal bit error rate corresponding to the intersection point, calculating the Q-factor corresponding to the determined ideal bit error rate.